Optical scanning device

ABSTRACT

The invention relates to an optical scanning device ( 1 ) for scanning an information layer ( 2 ) by means of a radiation beam ( 4 ). The device includes: a radiation source ( 6 ) for providing said radiation beam, a lens system ( 7 ) for transforming said radiation beam to a converging beam ( 16 ), and a wavefront modifier including a first element ( 301 ) and a second element ( 302 ) having each an aspheric surface and being mutually movable in a plane perpendicular to an optical axis ( 12 ) of said lens system for introducing a wavefront modification (W 1 ) in said converging beam. According to the invention, the aspheric surfaces of said first and second elements are shaped so that a mutual rotational displacement of the elements about an axis of rotation (Z A ) which is parallel to said optical axis ( 12 ) generates said wavefront modification (W 1 ).

The invention relates to an optical scanning device for scanning aninformation layer of an optical record carrier by means of a radiationbeam, including: (a) a radiation source for providing said radiationbeam, (b) a lens system for transforming said radiation beam to aconverging radiation beam so as to form a scanning spot in the positionof the information layer, the lens system including a first objectivelens having an optical axis, and (c) a wavefront modifier arrangedbetween said radiation source and the position of said scanning spot,the wavefront modifier including a first element having a first asphericsurface and a second element having a second aspheric surface, saidfirst and second elements being mutually movable in a planeperpendicular to said optical axis for introducing a modification W₁ insaid converging beam.

The invention also relates to a wavefront modifier for transforming afirst radiation beam into a second radiation beam, the wavefrontmodifier having an optical axis and including a first element having afirst aspheric surface and a second element having a second asphericsurface, said first and second elements being mutually movable in aplane perpendicular to said optical axis for introducing a wavefrontmodification W₁ in said second radiation beam.

A “wavefront modifier” is used for introducing a “wavefrontmodification”, that is for modifying the shape of the wavefront of aradiation beam by introducing path length differences in dependence onthe position in the cross-section of the beam. A wavefront modificationmay be of a first, second, etc. order of a radius in the cross-sectionof the radiation beam if the mathematical function describing thewavefront modification has a radial order of three, four, etc.,respectively. Wavefront tilt or distortion is an example of a wavefrontmodification of the first order. Astigmatism and curvature of field anddefocus are two examples of a wavefront modification of the secondorder. Coma is an example of a wavefront modification of the thirdorder. Spherical aberration is an example of a wavefront modification ofthe fourth order. It is noted that some wavefront modifications, such aswavefront tilt, astigmatism and coma, are dependent on a direction inthe cross-section of the radiation beam. Some wavefront modifications,such as defocus and spherical aberration, are independent on a directionin the cross-section of the radiation beam. For more information on themathematical functions representing the aforementioned wavefrontmodifications, see, e.g. the book by M. Born and E. Wolf entitled“Principles of Optics,” pp. 464–470 (Pergamon Press 6^(th) Ed.) (ISBN0-08-026482-4).

The wavefront modifier may be used for changing properties of theradiation beam such as its vengeance by introducing a focus curvature inthe wavefront of the beam or to change the direction of the beam byintroducing tilt. A wavefront modifier may also operate as a wavefrontcompensator for compensating an undesired shape of the wavefront of theradiation beam, e.g. for removing an optical aberration such asspherical aberration or coma from the wavefront of the radiation beam.

“Scanning an information layer” refers to scanning by a radiation beamfor: reading information from the information layer (“reading mode”),writing information in the information layer (“writing mode”), and/orerasing information from the information layer (“erase mode”).“Information density” refers to the amount of stored information perunit area of the information layer. It is determined by, inter alia, thesize of the scanning spot formed by the scanning device on theinformation layer to be scanned. The information density may beincreased by decreasing the size of the scanning spot. Since the size ofthe spot depends, inter alia, on the wavelength λ and the numericalaperture NA of the radiation beam forming the spot, the size of thescanning spot can be decreased by increasing NA and/or by decreasing λ.

When scanning an optical record carrier having the shape of a disc withan optical scanning device of the type described in the openingparagraph, a problem is the generation of coma in the converging beamdue to a warpage of the disc in the radial direction of the disc. Suchwarpage results in the presence of a tilt between the optical axis ofthe objective lens and the normal direction of the disc. This problem iseven more critical in case of record carriers having high informationdensity, where the numerical aperture of the radiation beam incident onthe record carrier is relatively high. For instance, this is the casefor record carriers of the so-called DVD+RW format, where the numericalaperture of the incident beam approximately equals 0.65.

A solution to said problem of generation of coma consists in using awavefront modifier arranged in the optical path of the light between theradiation source and the position of the scanning spot, the modifiercomprising a pair of plates having each a flat surface and an asphericsurface. Such a modifier is known from the article by I. Palusinski etal entitled “Lateral shift variable aberration generators”, AppliedOptics Vol. 38 (1999) pp. 86–90. The plates are complementary such thatwhen mated they form a flat plate having no optical power. A mutuallinear displacement of the two plates in one direction perpendicular tothe optical axis of the lens system results in the generation of awave-front deformation which depends on the linear displacement and theshape of the aspheric surfaces.

An object of the invention is to provide an optical scanning deviceincluding a wavefront modifier which is an alternative to the onedescribed in said article by Palusinski, for correcting the wavefrontmodification in the converging beam incident on the record carrier to bescanned.

This object is achieved by the optical scanning device as described inthe opening paragraph wherein, according to the invention, said firstand second aspheric surfaces are shaped so that a mutual rotationaldisplacement of said first and second elements over an angle of rotationabout an axis of rotation which is parallel to said optical axisgenerates said wavefront modification.

An advantage of generating the second wavefront modification by arotational displacement of such elements is that the construction of thewavefront modifier is simple, robust and cheap. The construction of arotational displacement about one axis of rotation requires, forinstance, only one elastic foil, one magnet and one coil, as opposed tothe known wavefront modifier wherein a linear displacement along areference direction must be realized.

In a preferred embodiment of the optical scanning device, the shapes ofsaid first aspheric surface is defined by a function S which includes:

the term “(y+R)(x²+(y+R)²−yR)” in order to introduce coma along theX_(O)-axis in said converging radiation beam (16),

the term

$``{{{- {R\left( {{2\left( {x^{2} + \left( {y + R} \right)^{2}} \right)} + R^{2}} \right)}}{\arctan\left( \frac{y + R}{x} \right)}} - \left( {x^{2} + \left( {y + R} \right)^{2} + {3R^{2}}} \right) + {{Rx}\left( {y + R} \right)}}"$in order to introduce coma along the Y_(O)-axis in said convergingradiation beam (16),

the term “y+R” in order to introduce tilt along the X_(O)-axis in saidconverging radiation beam (16),

the term “x” in order to introduce tilt along the Y_(O)-axis in saidconverging radiation beam (16),

the term

$``{{\left( {x^{2} + \left( {y + R} \right)^{2}} \right){\arctan\left( \frac{y + R}{x} \right)}} + {2{Rx}}}"$in order to introduce defocus aberration in said converging radiationbeam (16),

the term

$``{{\left( {x^{2} + \left( {y + R} \right)^{2}} \right){\arctan\left( \frac{y + R}{x} \right)}} + {x\left( {y + R} \right)}}"$in order to introduce astigmatism along the X_(O)-axis in saidconverging radiation beam (16),

the term

$``{{\left( {x^{2} + \left( {y + R} \right)^{2}} \right){\arctan\left( \frac{y + R}{x} \right)}} - {x\left( {y + R} \right)}}"$in order to introduce astigmatism along the Y_(O)-axis in saidconverging radiation beam (16),

the term

$``{{\left( {\left( {x^{2} + \left( {y + R} \right)^{2}} \right)^{2} + {4{R^{2}\left( {x^{2} + \left( {y + R} \right)^{2}} \right)}} + R^{4}} \right){\arctan\left( \frac{y + R}{x} \right)}} + {4{R\left( {\left( {x^{2} + \left( {y + R} \right)^{2}} \right) + R^{2}} \right)}x} - {2R^{2}{x\left( {y + R} \right)}}}"$in order to introduce spherical aberration in said converging radiationbeam (16),

the term

$``{{{- \frac{2}{3}}x^{3}} - {x\left( {y + R} \right)}^{2}}"$in order to introduce line coma along the Y_(O)-axis in said convergingradiation beam (16),

the term

$``{{x^{2}\left( {y + R} \right)} + {\frac{2}{3}\left( {y + R} \right)^{3}}}"$in order to introduce line coma along the X_(O)-axis in said convergingradiation beam (16),where “(x, y)” are the Cartesian coordinates in the direct orthogonalsystem X_(O)Y_(O) in said reference plane (X_(A)Y_(A)) and having itsorigin on the second point of intersection (O) of said optical axis (12)and said reference plane, the Y_(O)-axis passing through said firstpoint of intersection (A), and “R” is the distance between said firstpoint of intersection (A) and said second point of intersection (A).

An advantage of providing the function S with the term

$``{{{- {R\left( {{2\left( {x^{2} + \left( {y + R} \right)^{2}} \right)} + R^{2}} \right)}}{\arctan\left( \frac{y + R}{x} \right)}} - \left( {x^{2} + \left( {y + R} \right)^{2} + {3R^{2}}} \right) + {{Rx}\left( {y + R} \right)}}"$is to generate coma in the Y_(O)-axis which can be used, e.g., forcompensating coma generated along the radial direction by a tilt betweenthe normal direction of the record carrier and the optical axis of theobjective lens. This provides the optical device with larger toleranceto disc tilt.

An advantage of shaping the function S with the term

$``{{\left( {\left( {x^{2} + \left( {y + R} \right)^{2}} \right)^{2} + {4{R^{2}\left( {x^{2} + \left( {y + R} \right)^{2}} \right)}} + R^{4}} \right){\arctan\left( \frac{y + R}{x} \right)}} + {4{R\left( {\left( {x^{2} + \left( {y + R} \right)^{2}} \right) + R^{2}} \right)}x} - {2R^{2}{x\left( {y + R} \right)}}}"$is to generate spherical aberration which can be used, e.g., forcompensating spherical aberration generated due to the difference inthickness between the transparent layers for a dual layer system of theso-called DVR format. This provides the optical device with largertolerance to spherical aberration.

Another object of the invention is to provide a wavefront modifier whichis an alternative to the one described in said article by Palusinski,for introducing a wavefront modification in the radiation beam emergingfrom the wavefront modifier.

This object is achieved by the wavefront modifier as described in theopening paragraph wherein, according to the invention, said first andsecond aspheric surfaces are shaped so that a mutual rotationaldisplacement of said first and second elements about an axis of rotationwhich is parallel to said optical axis generates said modification.

The objects, advantages and features of the invention will be apparentfrom the following, more detailed description of the invention, asillustrated in the accompanying drawings, in which:

FIG. 1 shows a scanning device according to the invention,

FIGS. 2 and 3 show first and second top views of a component of thescanning device shown in FIG. 1, in a first position and in a secondposition, respectively,

FIG. 4 shows a first cross-section of the component shown in FIG. 2,along a line I—I shown in FIG. 2,

FIG. 5 shows a second cross-section of the component shown in FIG. 3,along a line II—II shown in FIG. 3,

FIG. 6 is a detailed view of the component shown in FIG. 2, and

FIG. 7 shows an alternative of components shown in FIG. 4.

FIG. 1 shows an optical scanning device 1 according to the invention,which is used scanning a first information layer 2 of a first opticalrecord carrier 3 with a first radiation beam 4.

The record carrier 3 comprises a transparent layer 5, one side of whichis provided with the information layer 2. The side of the informationlayer 2 facing away from the transparent layer 5 may be protected fromenvironmental influences by a protective layer. The transparent layer 5acts as a substrate for the record carrier 3 by providing mechanicalsupport for the information layer 2. Alternatively, the transparentlayer 5 may have the sole function of protecting the information layer2, while the mechanical support is provided by a layer on the other sideof the information layer 2, for instance by the protective layer or byan additional information layer and transparent layer connected to theinformation layer 2. The information layer 2 is a surface of the recordcarrier 3 that contains tracks. A track is a path to be followed by afocused or converging radiation beam on which path optically-readablemarks that represent information are arranged. In the following, thereference “T” designates such a track. The marks may be, e.g., in theform of pits or areas having a reflection coefficient or a direction ofmagnetization different from the surroundings. With reference to FIG. 1and seq. and in the case where the record carrier 3 has the shape of adisc having a center C and includes tracks that are substantiallycircular with the center C, “Y” is the reference axis parallel to the“radial direction,” that is, the direction between the center C and apoint of a track to be scanned, and “X” is the reference axis parallelto the “tangential direction,” that is, the direction that is tangentialto the track and perpendicular to the “radial direction” in the plane ofthe disc. Also with reference to FIG. 1 et seq., “Z” is the referenceaxis of an optical axis 12 of the optical scanning device 1. It is notedthat (X, Y, Z) is a direct orthogonal coordinate system, when the disc 3is parallel to the plane XY.

By way of illustration only, in the case where the optical recordcarrier 3 is a disc of the so-called DVR-format, the thickness of thetransparent layer 5 approximately equals 0.1 mm. Alternatively, in thecase where the record carrier 4 is a disc of the so-called DVD-format,the thickness of the transparent layer 5 approximately equals 0.6 mm.

The optical scanning device 1 includes a radiation source 6, a lenssystem 7 having an optical axis 12, and a wavefront modifier 30. Thedevice 1 further includes a beam splitter 8, a collimator lens 9, adetection system 10, a servosystem 11, a focus actuator (not shown inFIG. 1), a radial actuator (not shown in FIG. 1), and an informationprocessing unit 14 for error correction.

The radiation source 6 is arranged for supplying the radiation beam 4for scanning the information layer 2 of the record carrier 3.Preferably, the radiation source 6 includes at least a semiconductorlaser that emits the radiation beam 4 at a selected wavelength λ By wayof illustration only, the wavelength λ preferably equals 405 and 660 nmin the case where the record carrier 3 is a DVR-format disc and aDVD-format disc, respectively. Furthermore, the radiation source 6 maybe provided with a grating structure (not shown in FIG. 1) for forming afirst satellite radiation beam and a second satellite radiation beam(which are not shown in FIG. 1) as the −1 and +1 order diffractedradiation beams from the central radiation beam 4.

The beam splitter 8 reflects the radiation beam 4 toward the collimatorlens 9. Preferably, the beam splitter 8 is formed by a plane parallelplate that is tilted with respect to the optical axis 12.

The collimator lens 9 transforms the radiation beam 4 to a collimatedradiation beam 14.

The lens system 7 transforms the collimated beam 14 into a convergingradiation beam 16 so as to form a scanning spot 17 in the position ofthe information layer 2. The converging beam 16 has a numerical apertureNA. By way of illustration only, in the case where the optical recordcarrier 3 is a disc of the so-called DVR-format, the numerical apertureNA of the converging beam 16 approximately equals 0.85 for both thereading mode and the writing mode. In the case where the optical recordcarrier 3 is a disc of the so-called DVD-format, the numerical apertureNA of the converging beam 16 approximately equals 0.60 for the readingmode and 0.65 for the writing mode.

The lens system 7 includes a first objective lens 18 having an entrancesurface 18 a and an exit surface 18 b. The lens system 7 may furtherinclude a second objective lens (not shown in FIG. 1), preferably in thecase where the numerical aperture NA approximately equals 0.85. Thesecond objective lens, together with the objective lens 18, forms adoublet-lens system that advantageously has a larger tolerance in mutualposition of the optical elements than a single-lens system formed onlyby the objective lens 18. The second objective lens is formed by aplano-convex lens having a convex surface that faces the objective lens18 and a flat surface that faces the position of the information layer2. Furthermore, the entrance surfaces and/or exit surfaces of the firstand/or second objective lens(es) are preferably aspherically curved forcompensating, e.g., spherical aberration, by using a process known from,e.g., the article by B. H. W. Hendriks and P. G. J. M. Nuyens entitled“Designs and manufacturing of far-field high NA objective lenses foroptical recording,” 413–414, SPIE 3749 (1999). It is noted that otherkinds of wavefront modification can be corrected by designing asphericallenses. However, such a correction depends on parameters that have beenpredetermined when designing the lenses; it remains the sameirrespective of the actual configuration of the components of theoptical scanning device 1, as opposed to the servo correction introducedby the wavefront modifier 30 (see below).

During scanning, the forward converging radiation beam 16 reflects onthe information layer 2, thereby forming a backward diverging radiationbeam 21 which returns on the optical path of the forward convergingradiation beam 16. The lens system 7 transforms the backward radiationbeam 21 to a backward collimated radiation beam 22. The collimator lens9 transforms such a backward collimated radiation beam to a backwardnon-collimated radiation beam 23. The beam splitter 8 separates theforward radiation beam 4 from the backward radiation beam 23 bytransmitting at least part of the backward radiation beam 23 towards thedetection system 10.

The detection system 10 is arranged for capturing said part of thebackward radiation beam 23 and converting it into one or more electricsignals. One of the signals is an information signal S_(data), the valueof which represents the information scanned from the information layer2. The information signal S_(data) may be processed by the informationprocessing unit 14 for error correction of the information extractedfrom the information layer 2. Other signals from the detection system 10are a focus error signal S_(focus) and a radial tracking error signalS_(radial). The value of the signal S_(focus) represents the axialdifference in height along the optical axis 12 between the scanning spot12 and the information layer 2. The signal S_(focus) is formed by thecommonly used “astigmatic method” which is known from, inter alia, thebook by G. Bouwhuis, J. Braat, A. Huijser et al, “Principles of OpticalDisc Systems,” pp. 75–80 (Adam Hilger 1985) (ISBN 0-85274-785-3). Thesignal S_(focus) is used for maintaining the scanning spot 17 in focusin the information layer 2. The value of the signal S_(radial)represents the distance in the plane of the information layer 2 betweenthe scanning spot 17 and the center of a track in this information layerto be followed by this scanning spot. The signal S_(radial) is formed bythe commonly used “radial push-pull method” which is known from, interalia, said book by G. Bouwhuis et al., pp. 70–73. The signal S_(radial)is used for maintaining the scanning spot 17 on track in the informationlayer 2.

The servosystem 11 is arranged for, in response to the signals S_(focus)and S_(radial), providing control signals S_(control) for controllingthe focus actuator and the radial actuator, respectively. The focusactuator controls the positions of the lens system 7 in a direction 25parallel to the optical axis 12 (axis Z), thereby controlling theposition of the scanning spot 17 such that it coincides substantiallywith the plane of the information layer 2. The radial actuator controlsthe positions of the lens system 7 in a direction 26 parallel to theradial direction (axis Y), thereby controlling the radial position ofthe scanning spot 17 such that it coincides substantially with thecenter line of the track to be followed in the information layer 2.

The wavefront modifier 30 is arranged between the radiation source 6 andthe position of the record carrier 3 for introducing a modification W₁in the converging beam 16. The wavefront modifier 30 including a pair ofelements (not shown in FIG. 1 but shown in FIG. 2 et seq.) formed by afirst element and a second element. The first and second elements have afirst aspheric surface and a second aspheric surface elements (not shownin FIG. 1 but shown in FIGS. 4 and 5). The first and second elements aremutually movable in a plane perpendicular to the optical axis 12 forintroducing the wavefront modification W₁ in the converging beam 16.

According to a first aspect of the invention, the first and secondaspheric surfaces are shaped so that a mutual rotational displacement ofthe first and second elements about an axis of rotation Z_(A) (not shownin FIG. 1 but shown in FIG. 2) which is parallel to the optical axis 12generates the wavefront modification W₁. Furthermore, the shape of thefirst aspheric surfaces is defined by a function S_(a)(r, θ) and theshape of the second aspheric surface is defined by a function S_(b)(r,θ), the function S_(a)(r, θ) and S_(b)(r, θ) being determined by:

$\begin{matrix}{{W_{1}\left( {r,\theta} \right)} \approx {{\left( {n_{a} - 1} \right)\varphi_{a}\frac{\partial S_{a}}{\partial\theta}} - {\left( {n_{b} - 1} \right)\varphi_{b}\frac{\partial S_{b}}{\partial\theta}}}} & \left( {1a} \right)\end{matrix}$where “(r, θ)” are polar coordinates in a reference plane X_(A)Y_(A)perpendicular to the optical axis 12, these coordinates being centeredon the first point of intersection A of the axis of rotation Z_(A) andthe reference plane X_(A)Y_(A), “W₁(r, θ)” is the wavefront modificationexpressed in the polar coordinates (r, θ), “n_(a)” is the refractiveindex of the first element and “n_(b)” is the refractive index of thesecond element, “Φ_(a)” is said angle of rotation of the first elementand “Φ_(b)” is said angle of rotation of the second element, and“S_(a)(r, θ)” represents the shape of the first aspheric surface and“S_(b)(r, θ)” represents the shape of the second aspheric surface.

According to another aspect of the invention, the first and secondaspheric surfaces are shaped so that a mutual rotational displacement ofthe first and second elements about an axis of rotation Z_(A) (not shownin FIG. 1 but shown in FIG. 2) which is parallel to the optical axis 12generates the wavefront modification W₁. Furthermore, the shapes of thefirst and second aspheric surfaces are substantially identical and theshape of said first aspheric surface is defined by a function S(r, θ)determined by:

$\begin{matrix}{{W_{1}\left( {r,\theta} \right)} \approx {\left( {n - 1} \right)\varphi\frac{\partial S}{\partial\theta}}} & \left( {1b} \right)\end{matrix}$where “(r, θ)” are polar coordinates in a reference plane X_(A)Y_(A)perpendicular to the optical axis 12, these coordinates being centeredon the first point of intersection A of the axis of rotation Z_(A) andthe reference plane X_(A)Y_(A), “W₁(r, θ)” is the wavefront modificationexpressed in the polar coordinates (r, θ), “n” is the refractive indexof the first and second elements, “Φ” is said angle of rotation, and“S(r, θ)” represents the shape of the first aspheric surface.

It is noted that, in the following, the shape of a given surface is“substantially defined” by a function S means that the actual shapeS_(actual) of the surface meets the following condition:0.9S<S _(actual)<1.1S.Preferably, the actual shape S_(actual) of the surface meets thefollowing condition:0.95S<S _(actual)<1.05S.More preferably, the actual shape S_(actual) of the surface meets thefollowing condition:0.99S<S _(actual)<1.01S.

By way of illustration only, in the embodiment of the optical scanningdevice 1 shown in FIG. 1, the wavefront modifier 30 is used forintroducing the wavefront modification W₁ in the collimated beam 14 inorder to compensate an amount of coma W₂ that is present in theconverging beam 16 due to a tilt of the record carrier 3. It is notedthat the presence of coma in the converging beam means that coma ispresent in the radiation beam traversing the transparent layer 5, fromthe surface 5 a of the record carrier 3 to the scanning spot 17. Thus,in this embodiment, the optical scanning device 1 includes a comacompensator 19 which includes a coma detector 33, a control circuit 31,and the wavefront modifier 30.

The coma detector 33 provides a detection signal 35 representative ofthe amount of coma W₂. In this embodiment, the coma detector 33 is atilt detector 33 and the detection signal 35 is a tilt signal. The tiltdetector 33 emits a radiation beam 34 towards the record carrier 3 anddetects the angle of the radiation beam reflected by the record carrier3. The position of the spot of the reflected beam in the plane is ameasurement for the angle and, hence, for the tilt of the record carrier3. The measured value of the tilt is directly proportional to the amountof coma W₂. The tilt detector 33 transforms that measured value into thetilt signal 35. It is noted that the tilt detector 33 maybe of any type.An alternative of the tilt detector 33 shown in FIG. 1 is a tiltdetector formed as a part of the control circuit 31, wherein the tiltsignal is derived from a combination of output signals of the detectionsystem 10.

The control circuit 31 is arranged for, responsive to the tilt signal35, providing control signals 32 for controlling the wavefront modifier30.

The wavefront modifier 30 is arranged, in this embodiment, in theoptical path of the collimated beam 14, between the collimator lens 9and the lens system 7. Thus, the wavefront modifier 30 transforms thecollimated beam 14 to a radiation beam 15 by introducing, in response tothe tilt signal 35, the wavefront modification W₁ in order to compensatethe amount of coma W₂. In other words, the wavefront modifier 30 isarranged so that:W ₁ +W ₂=0.  (2)

FIGS. 2 and 3 show a first top view and a second top view of anembodiment of the wavefront modifier 30 shown in FIG. 1, seen from theside of the collimator lens 9. FIG. 4 shows a first cross-section of thewavefront modifier 30, seen along a line I—I shown in FIG. 2. FIG. 5shows a second cross-section of the wavefront modifier 30, seen along aline II—II shown in FIG. 2.

As shown in FIGS. 2 through 5, the wavefront modifier 30 includes thefirst and second elements which are formed, in this embodiment, by afirst plate 301 and a second plate 302, respectively. The wavefrontmodifier 30 also includes a body 50 for supporting the plates 301 and302. As shown in FIG. 2, the wavefront modifier 30 further includes ahinge 51 for enabling, by means of control means 52, the rotationaldisplacement about the axis Z_(A) between, in this embodiment, the body50 and the plate 301.

It is noted that, in FIGS. 2 and 4, the plates 301 and 302 mate eachother so as to form a plane parallel plate. In FIGS. 3 and 5, there is arotational displacement between the two plates about the axis ofrotation Z_(A), that is, in this embodiment, the plate 301 is rotatedabout the axis Z_(A) and the plate 302 is stationary.

The plate 301 has an entrance surface 301 a facing the collimator lens 9and an exit surface 301 b facing the plate 302. The exit surface 301 bis aspherically curved (as described below). The entrance surface 301 ais, in this embodiment, substantially plane. It is noted that the planeof the entrance surface 301 a is defined, in this embodiment, as thereference plane X_(A)Y_(A) and “A” is the point of intersection of theaxis of rotation Z_(A) and the plane entrance surface 301 a. It is alsonoted that (A, X_(A), Y_(A), Z_(A)) are two direct orthogonal base.

The plate 302 (shown in dotted line in FIG. 3) has an entrance surface302 a facing the exit surface 301 b of the plate 301 and an exit surface302 b facing the objective lens 18. The entrance surface 302 a isaspherically curved (as described below). The exit surface 302 b is, inthis embodiment, substantially plane, parallel to the X_(A)-axis and theY_(A)-axis.

It is noted that, in this embodiment, said first and second asphericsurfaces are formed by the exit surface 301 b and the entrance surface302 a. It is also noted that the aspheric surfaces 301 b and 302 a havethe same shape for any value of the polar coordinates (r, θ).

By way of illustration only, the plates 301 and 302 can be made ofplastic, e.g. the material commonly known in the commerce under thedesignation PMMA, where the optical index equals, e.g., 1.5066.

The body 50 has four inner walls 50 a through 50 d arranged so as toform an opening through the body 50 in which the plates 301 and 302 areprovided as explained below. By way of illustration, the body 50 is madeof aluminum.

FIG. 6 is a detailed view of the hinge 51 that includes, in thisembodiment, a foil 53, a first support 54 fixed to the plate 301 and asecond support 55 fixed to the body 50 (on the wall 50 a, in theembodiment shown in FIGS. 2 and 3). The foil 53 has a first portion 53 aengaged with the support 54, a second portion 53 b engaged with thesupport 55, and a third portion 53 c that is pivotable about the axisZ_(A). The foil 53 is made of a material having elasticity properties,e.g., spring steel so that the third portion 53 c, together with theplate 301, can pivot or rotate about the axis Z_(A). Preferably, asshown in FIGS. 2 and 3, the axis of rotation Z_(A) is eccentric withrespect to the optical axis 12. More preferably, also as shown in FIGS.2 and 3, the axis of rotation Z_(A) is outside of the position of thecross-section 140 of the collimated beam 14 that is incident on theentrance surface 301 a of the plate 301. By way of illustration only,the distance R between the point A and the center O equals to 3.6 mm.

In the embodiment shown in FIGS. 2 and 3, the control means 52 areformed by a coil 60 provided with the plate 301 and by an electromagnet61 provided with the body 50 (in the wall 50 c opposite to the wall 50a) and controlled by the control signals 32 of the control circuit 31.

In the configuration of the plates 301 and 302 shown in FIGS. 2 and 4,the plates 301 and 302 mate each other. There is a first gap between theplate 301 and the body 50, with a height “d” which is substantiallyconstant, as shown in FIG. 4. By way of illustration only, the height dis typically equal to 0.3 mm. It is noted, in this configuration, thatthe total thickness D, i.e. the sum of the thickness of the plate 301,the first gap, and the thickness of the plate 302 along the Z_(A)-axis,is substantially constant. By way of illustration only, the totalthickness D approximately equals 2 mm. There is also a second gapbetween the plate 301 and the plate 302, with a height “h” along theZ_(A)-axis which, in this configuration, equals a substantially constantvalue, h_(O). The value of the height h is chosen as explained below. Itis noted that, in this configuration, the positions of the asphericsurfaces 302 a and 301 b in the base (A, X_(A), Y_(A), Z_(A)) (shown inFIG. 2) are given by S₁(x, y) and h+S₁(x, y), respectively.

In the configuration of the plates shown in FIGS. 3 and 5, there is amutual rotational displacement of the plates 301 and 302 over the angleΦ as about the axis Z_(A) as shown in FIG. 3. In this embodiment, theplate 301 is rotated by the angle Φ and the plate 302 is stationary,that is, remains in the same position than in the configuration shown inFIG. 2. The value of the angle Φ is chosen as explained below. It isnoted, in this configuration, that the height h between the plates 301and 302 is no longer substantially constant, because of the asphericityof the exit surface 301 b and the entrance surface 302 a. This resultsin different optical paths for the radiation beam emerging from the exitsurface 301 b of the plate 301. As a result, in this embodiment, thewavefront modification W₁ is introduced in the radiation beam 15 forcorrecting coma in the converging beam 16.

During scanning of a track T of the record carrier 3 and with referenceto FIG. 2, the optical scanning device 1 can be oriented with respect tothe record carrier 3 so that the radial direction (Y) of the track T isparallel to the Y_(A)-axis and the tangential direction (X) of the trackT is parallel to the X_(A)-axis. Thus, the wavefront modifier 30 canintroduce, in the embodiment shown in FIG. 1, the amount of coma W₂ inthe radial direction (Y) or the tangential direction (X) of the track T.As a example only, the design of the aspheric surfaces 301 b and 302 ais now described in relation to the embodiment of the optical scanningdevice 1 for compensating the amount of coma W₂ in the tangentialdirection (X) of the track T. The function S is designated, in thisexample, by the reference “S₁”.

The amount of coma W₂ along the tangential direction (X) can berepresented as follows:W ₂(x, y)=A ₁ x(x ² +y ²)  (3)where “(x, y)” are the Cartesian coordinates in the direct orthogonalsystem X_(O)Y_(O) in the reference plane X_(A)Y_(A) and having itsorigin on the second point of intersection O of the optical axis 12 andthe reference plane X_(A)Y_(A), the Y_(O)-axis passing through the firstpoint of intersection A, and “A₁” is a parameter which is constant interms of (x, y) and which depends on the value of the tilt angle of thedisc-shaped record carrier 3. It is also noted that (O, X_(O), Y_(O),Z_(O)) are two direct orthogonal base.

When substituting Equation (3) in Equation (2), it is found in the (O,X_(O), Y_(O), Z_(O)) base that:W ₁(x, y)=−A ₁ x(x ² +y ²)  (4)

After substituting Equation (1b) in Equation (4), it is found that thefunction S₁ associated with the aspheric surface 301 b is given by:S ₁(r,θ)=B ₁ r sin θ(r ² −Rr sin θ+R ²)+f₁(r)  (5)where “B₁” is a constant parameter defined by:

$\begin{matrix}{B_{1} = \frac{- A_{1}}{\left( {n - 1} \right)\varphi}} & (6)\end{matrix}$“f₁(r)” is a function of the polar coordinate r, that is symmetricalwith respect to the Z_(A)-axis, and “R” is the distance between thepoints of intersection A and O and has a sign determined by:R>0, if {right arrow over (OA)}.{right arrow over (v)}<0 orR≦0, if {right arrow over (OA)}.{right arrow over (v)}≧0wherein {right arrow over (v)} is a unitary vector of the Y_(O)-axis.

Expressed in the polar coordinates (r, θ) in Equation (5), it is foundthat the function S₁ can also be expressed in the Cartesian coordinates(x, y) with respect to (O, X_(O), Y_(O), Z_(O)) as follows:S ₁(x, y)=B ₁(y+R)(x ²+(y+R)² −yR)+f₁(√{square root over (x²+(y+R)²)})  (7)

Thus, the shapes of the aspheric surface 301 b and 302 a can bedesigned, in this example, by knowing Equation (7) and choosing thevalues of the parameter B₁ and the distance R.

The choice of the distance R depends on the geometry of the collimatedbeam 14. By way of illustration only, the distance |R| may be chosenbetween 3 mm and 6 mm.

The choice of the value of the parameter B₁ depends on the parameter A,(n−1) and the angle Φ pursuant to Equation (6). Thus, it is noted that,considering a given amount of coma to be compensated, i.e. a given valueof the parameter A, there is a trade-off between the choice of thevalues of the parameter B₁ and of the angle Φ. For instance, if a largevalue of the parameter B₁ is chosen, the aspheric surface 301 b is thendesigned with a relatively large peak-to-peak value in height. Thisresults in an important curvature of the surface 301 a, thereby makingthe plate 301 difficult to rotate. By contrast, the choice of a largevalue of the angle Φ requires a rotational displacement of the plate 301in the body 50 with a large amplitude, thereby making the wavefrontmodifier 30 difficult to make. By way of illustration only, the valuesof the angle Φ is comprised between −3 and +3 degrees.

Regarding the mutual positioning of the aspheric surfaces 301 b and 302a, the value h₀ of the height h (shown in FIG. 4) must be chosen. It isnoted that the choice of the value h₀ is dependent on the rotation ofthe plate 301 over the angle Φ and the parameter B₁. Thus, a large valueh₀ allows the plate 301 to rotate without being into contact with thestationary plate 302. However, it is noted that the rotation of theplate 301 over the angle Φ also generate an amount of astigmatism W₃that depends on the height of the gap between the plates 301 and 302.Ray-tracing simulations have been made from Equation (7) with differentvalues h₀. The results of these simulations are shown in Table 1 below.Table 1 shows the root-mean-square values W_(1,rms) and W_(3,rms) of theamount of coma W₁ and the amount of astigmatism W₃ for the differentvalues h₀, in the case where the shapes of the aspheric surfaces 301 band 302 a are defined by the function S₁ according to Equation (5) andunder the following conditions: Φ=2 degrees; R=3.6 mm; B₁=0.004 mm⁻²;φ=3 mm; and λ=405 mm, where “φ” and “λ” are the diameter and thewavelength of the collimated beam 14, respectively. It is noted thatcoma and astigmatism have been expressed in the form of the Zernikecoefficients as known, e.g., from said book by M. Born, pp. 469–470.

TABLE 1 h₀ (μm) W_(1, rms) (mλ) W_(3, rms) (mλ) 0 70 10 10 70 17 50 7043

Therefore, the value h₀ must be chosen sufficiently high such that theplate 301 can rotate without being into contact with the stationaryplate 302. It must also be sufficiently low so that the rotation of theplate 301 generates a low amount of astigmatism W₃. It has been foundthat, due to the rotation of the plates 301 and 302, the height h₀ mustbe higher than 0.6 μm.

It is to be appreciated that numerous variations and modifications maybe employed in relation to the embodiments described above, withoutdeparting from the scope of the invention which is defined in theappended claims.

In particular, the wavefront modifier 30 shown in FIGS. 1 through 6 maybe adapted for modifying a wavefront modification other than coma in thetangential direction (X). For instance, as an alternative to the opticalscanning device shown in FIG. 2, the aspheric surfaces 301 b and 302 bare shaped so as to generate the wavefront modification W₁ forcompensating the coma aberration W₂(x, y) (=A₂y(x²+y²)) along theY_(O)-axis (radial direction). The shapes of the aspheric surfaces aresubstantially defined by a function S₂ given by:

$\begin{matrix}{{S_{2}\left( {x,y} \right)} = {{{- B_{2}}{R\left( {{2\left( {x^{2} + \left( {y + R} \right)^{2}} \right)} + R^{2}} \right)}{\arctan\left( \frac{y + R}{x} \right)}} -}} \\{{B_{2}\left( {x^{2} + \left( {y + R} \right)^{2} + {3R^{2}}} \right)} + {B_{2}{{Rx}\left( {y + R} \right)}} +} \\{f_{2}\left( \sqrt{x^{2} + \left( {y + R} \right)^{2}} \right)}\end{matrix}$where “B₂” is a constant parameter in terms of the Cartesian coordinates(x, y) with respect to (O, X_(O), Y_(O), Z_(O)) and “f₂” is a functionof the Cartesian coordinates (x, y) with respect to (O, X_(O), Y_(O),Z_(O)) that is symmetrical with respect to the Z_(A) axis.

As another alternative to the optical scanning device shown in FIG. 2,the aspheric surfaces 301 b and 302 b are shaped so as to generate thewavefront modification W₁ for compensating the tilt aberration W₂(x, y)(=A₃x) along the X_(O)-axis (tangential direction). The shapes of theaspheric surfaces are substantially defined by a function S₃ given by:S ₃(x, y)=B ₃(y+R)+f₃(√{square root over (x ²+(y+R)²)})where “B₃” is a constant parameter in terms of the Cartesian coordinates(x, y) with respect to (O, X_(O), Y_(O), Z_(O)) and “f₃” is function ofthe Cartesian coordinates (x, y) with respect to (O, X_(O), Y_(O),Z_(O)) that is symmetrical with respect to the Z_(A) axis.

As another alternative to the optical scanning device shown in FIG. 2,the aspheric surfaces 301 b and 302 b are shaped so as to generate thewavefront modification W₁ for compensating the tilt aberration W₂(x, y)(=A₄y) along the Y_(O)-axis (radial direction). The shapes of theaspheric surfaces are substantially defined by a function S₄ given by:S ₄(x, y)=B ₄ x+f ₄(√{square root over (x ²+(y+R)²)})where “B₄” is a constant parameter in terms of the Cartesian coordinates(x, y) with respect to (O, X_(O), Y_(O), Z_(O)) and “f₄” is function ofthe Cartesian coordinates (x, y) with respect to (O, X_(O), Y_(O),Z_(O)) that is symmetrical with respect to the Z_(A) axis.

As another alternative to the optical scanning device shown in FIG. 2,the aspheric surfaces 301 b and 302 b are shaped so as to generate thewavefront modification W₁ for compensating the defocus aberration W₂(x,y) (=A₅(x²+y²)). The shapes of the aspheric surfaces are substantiallydefined by a function S₅ given by:

$\begin{matrix}{{S_{5}\left( {x,y} \right)} = {{{B_{5}\left( {x^{2} + \left( {y + R} \right)^{2}} \right)}{\arctan\left( \frac{y + R}{x} \right)}} + {2B_{5}{Rx}} +}} \\{f_{5}\left( \sqrt{x^{2} + \left( {y + R} \right)^{2}} \right)}\end{matrix}$where “B₅” is a constant parameter in terms of the Cartesian coordinates(x, y) with respect to (O, X_(O), Y_(O), Z_(O)) and “f₅” is function ofthe Cartesian coordinates (x, y) with respect to (O, X_(O), Y_(O),Z_(O)) that is symmetrical with respect to the Z_(A) axis.

As another alternative to the optical scanning device shown in FIG. 2,the aspheric surfaces 301 b and 302 b are shaped so as to generate thewavefront modification W₁ for compensating the astigmatism aberrationW₂(x, y) (=A₆x²) along the X_(O)-axis (tangential direction). The shapesof the aspheric surfaces are substantially defined by a function S₆given by:

$\begin{matrix}{{S_{6}\left( {x,y} \right)} = {{{B_{6}\left( {x^{2} + \left( {y + R} \right)^{2}} \right)}{\arctan\left( \frac{y + R}{x} \right)}} + {B_{6}{x\left( {y + R} \right)}} +}} \\{f_{6}\left( \sqrt{x^{2} + \left( {y + R} \right)^{2}} \right)}\end{matrix}$where “B₆” is a constant parameter in terms of the Cartesian coordinates(x, y) with respect to (O, X_(O), Y_(O), Z_(O)) and “f₆” is function ofthe Cartesian coordinates (x, y) with respect to (O, X_(O), Y_(O),Z_(O)) that is symmetrical with respect to the Z_(A) axis.

As another alternative to the optical scanning device shown in FIG. 2,the aspheric surfaces 301 b and 302 b are shaped so as to generate thewavefront modification W₁ for compensating the astigmatism aberrationW₂(x, y) (=A₇y²) along the Y_(O)-axis (radial direction). The shapes ofthe aspheric surfaces are substantially defined by a function S₇ givenby:

$\begin{matrix}{{S_{7}\left( {x,y} \right)} = {{{B_{7}\left( {x^{2} + \left( {y + R} \right)^{2}} \right)}{\arctan\left( \frac{y + R}{x} \right)}} - {B_{7}{x\left( {y + R} \right)}} +}} \\{f_{7}\left( \sqrt{x^{2} + \left( {y + R} \right)^{2}} \right)}\end{matrix}$where “B₇” is a constant parameter in terms of the Cartesian coordinates(x, y) with respect to (O, X_(O), Y_(O), Z_(O)) and “f₇” is function ofthe Cartesian coordinates (x, y) with respect to (O, X_(O), Y_(O),Z_(O)) that is symmetrical with respect to the Z_(A) axis.

As another alternative to the optical scanning device shown in FIG. 2,the aspheric surfaces 301 b and 302 b are shaped so as to generate thewavefront modification W₁ for compensating the spherical aberrationW₂(x, y) (=A₈(x²+y²)²). The shapes of the aspheric surfaces aresubstantially defined by a function S₈ given by:

$\begin{matrix}{{S_{8}\left( {x,y} \right)} = {B_{8}\left( {\left( {x^{2} + \left( {y + R} \right)^{2}} \right)^{2} + {4{R^{2}\left( {x^{2} + \left( {y + R} \right)^{2}} \right)}} + R^{4}} \right)}} \\{{\arctan\left( \frac{y + R}{x} \right)} + {4B_{8}{R\left( {\left( {x^{2} + \left( {y + R} \right)^{2}} \right) + R^{2}} \right)}x} -} \\{{2B_{8}R^{2}{x\left( {y + R} \right)}} + {f_{8}\left( \sqrt{x^{2} + \left( {y + R} \right)^{2}} \right)}}\end{matrix}$where “B₈” is a constant parameter in terms of the Cartesian coordinates(x, y) with respect to (O, X_(O), Y_(O), Z_(O)) and “f₈” is function ofthe Cartesian coordinates (x, y) with respect to (O, X_(O), Y_(O),Z_(O)) that is symmetrical with respect to the Z_(A) axis.

As another alternative to the optical scanning device shown in FIG. 2,the aspheric surfaces 301 b and 302 b are shaped so as to generate thewavefront modification W₁ for compensating the line coma aberrationW₂(x, y) (=A₉y³) along the Y_(O)-axis (radial direction). The shapes ofthe aspheric surfaces are substantially defined by a function S₉ givenby:

${S_{9}\left( {x,y} \right)} = {{{- \frac{2B_{9}}{3}}x^{3}} - {B_{9}{x\left( {y + R} \right)}^{2}} + {f_{9}\left( \sqrt{x^{2} + \left( {y + R} \right)^{2}} \right)}}$where “B₉” is a constant parameter in terms of the Cartesian coordinates(x, y) with respect to (O, X_(O), Y_(O), Z_(O)) and “f₉” is function ofthe Cartesian coordinates (x, y) with respect to (O, X_(O), Y_(O),Z_(O)) that is symmetrical with respect to the Z_(A) axis.

As another alternative to the optical scanning device shown in FIG. 2,the aspheric surfaces 301 b and 302 b are shaped so as to generate thewavefront modification W₁ for compensating the line coma aberrationW₂(x, y) (=A₁₀x³) along the X_(O)-axis (tangential direction). Theshapes of the aspheric surfaces are substantially defined by a functionS₁₀ given by:

${S_{10}\left( {x,y} \right)} = {{B_{10}{x^{2}\left( {y + R} \right)}} + {\frac{2B_{10}}{3}\left( {y + R} \right)^{3}} + {f_{10}\left( \sqrt{x^{2} + \left( {y + R} \right)^{2}} \right)}}$where “B₁₀” is a constant parameter in terms of the Cartesiancoordinates (x, y) with respect to (O, X_(O), Y_(O), Z_(O)) and “f₁₀” isfunction of the Cartesian coordinates (x, y) with respect to (O, X_(O),Y_(O), Z_(O)) that is symmetrical with respect to the Z_(A) axis.

It is noted that the functions S₁ through S₁₀ are not disclosed in saidarticle by Palusinski.

An alternative of the wavefront modifier 30 shown in FIGS. 3 and 5 isarranged so that the plate 302 is rotated by an angle +Φ about the axisZ_(A) and the plate 301 is stationary.

Another alternative of the wavefront modifier 30 shown in FIGS. 3 and 5is arranged so that the plate 302 is rotated by an angle +Φ about theaxis Z_(A) and the plate 301 is rotated by an angle −Φ about the axis ofrotation Z_(A) by means of an additional hinge provided with the plate302 similarly to the hinge described with reference to FIG. 6.

Another alternative of the wavefront modifier 30 shown in FIGS. 3 and 5is the aspheric surface 301 b and/or the aspheric surface 302 ainclude(s) at least a step-function Q(r, θ) which equals a nonzeroconstant parameter “q” for a portion of the corresponding asphericsurface, and zero for the remaining part of that surface. The parameter“q” is substantially equal to mλ/(n−1) where “λ” is the wavelength ofthe collimated beam 14, “m” is an integer value and “n” is therefractive index of the corresponding plate. Hence the correspondingplate is modified in a similar way as a Fresnel lens known, e.g., fromthe book by W. J. Smith, “Modern Optical Engineering”, pp. 257–258(McGrraw-Hill, 2d Ed.) (ISBN 0-07-059174-1)). It is noted that thefunctions S_(a)(r, θ) and S_(b)(r, θ) may also include such astep-function Q.

In an alternative of the body 50 shown in FIGS. 2 and 3, the plates 301and 302 are provided so that the point of intersection A is located inthe close vicinity of the inner wall 50 b, 50 c or 50 d of the body 50.The location of the point A can be chosen in order to make the functionthat defines the shape of the aspheric surfaces 301 b and 302 a moresimple.

In an alternative of the optical scanning device shown in FIG. 1, theobjective lens 18 may be formed by said first and second elements havingsaid first and second aspheric surfaces, respectively.

In another alternative of the optical scanning device shown in FIG. 1,the wavefront modifier 30 may be arranged in the optical path of thelight between the radiation source and the position of the scanning spotother than in the optical path of the collimated beam 14. It is notedthat the shapes of the plates 301 and 302 must be adapted to thedimensions of the radiation beam in the optical path of which thewavefront modifier is arranged. By way of illustration only, FIG. 7shows an alternative to the plates 301 and 302 shown in FIG. 4, in theform of the plates 301′ and 302′. As shown in FIG. 7, the plates 301′and 302′ are arranged in the optical path of a diverging radiation beamand the surfaces 301 a′, 301 b′, 302 a′ and 302 b′ are adapted to thevariable dimensions of the radiation beam along its axis of propagation.

As an improvement of the wavefront modifier show in FIG. 1, thewavefront modifier can be provided with a position detector which isknown from PH-N 17.844 and incorporated herein by reference. It is notedthat the wavefront modification W₁ will only compensate the wavefrontdistortion W₂ only if the wavefront modification W₁ is correctly centredwith respect to the optical axis 12 of the objective lens 18. Thecompensation is not correct if wavefront modification W₁ is centred onthe axis of the collimated beam 14 and if the objective lens 18 isdisplaced in the radial direction (Y) because of radial tracking.

Furthermore, the wavefront modifier 30 shown in FIGS. 1 through 6 may beused for modifying a wavefront modification for optical devices otherthan the optical scanning device 1 shown in FIG. 1. For instance, thewavefront modifier is suitable for a zoom lens; it generates a wavefrontmodification in the form of defocus in order to change the focal lengthof the zoom lens, thereby making the focal length adjustable.

1. An optical scanning device (1) for scanning an information layer (2)of an optical record carrier (3) by means of a radiation beam (4),including: a radiation source (6) for providing said radiation beam, alens system (7) for transforming said radiation beam to a convergingradiation beam (16) so as to form a scanning spot (17) in the positionof the information layer, the lens system including a first objectivelens (18) having an optical axis (12), and a wavefront modifier (30)arranged between said radiation source and the position of said scanningspot, the wavefront modifier including a first element (301) having afirst aspheric surface (301 b) and a second element (302) having asecond aspheric surface (302 a), said first and second elements beingmutually movable in a plane perpendicular to said optical axis forintroducing a wavefront modification W₁ in said converging beam,characterized in that said first and second aspheric surfaces are shapedso that a mutual rotational displacement of said first and secondelements over an angle of rotation (Φ) about an axis of rotation (Z_(A))which is parallel to said optical axis (12) generates said wavefrontmodification W₁.
 2. The optical scanning device (1) as claimed in claim1, wherein the shape of said first aspheric surface (301 b) is definedby a function S_(a)(r, θ) and the shape of said second aspheric surface(302 a) is defined by a function S_(b)(r, θ), the function S_(a)(r, θ)and S_(b)(r, θ) being determined by:${W_{1}\left( {r,\theta} \right)} \approx {{\left( {n_{a} - 1} \right)\varphi_{a}\frac{\partial S_{a}}{\partial\theta}} - {\left( {n_{b} - 1} \right)\varphi_{b}\frac{\partial S_{b}}{\partial\theta}}}$where “(r, θ)” are polar coordinates in a reference plane X_(A)Y_(A)perpendicular to the optical axis 12, these coordinates being centeredon the first point of intersection A of the axis of rotation Z_(A) andthe reference plane X_(A)Y_(A), “W₁(r, θ)” is the wavefront modificationexpressed in the polar coordinates (r, θ), “n_(a)” is the refractiveindex of the first element and “n_(b)” is the refractive index of thesecond element, “Φ_(a)” is said angle of rotation of the first elementand “Φ_(b)” is said angle of rotation of the second element, and“S_(a)(r, θ)” represents the shape of the first aspheric surface and“S_(b)(r, θ)” represents the shape of the second aspheric surface. 3.The optical scanning device (1) as claimed in claim 1, wherein theshapes of said first and second aspheric surfaces (301 b, 302 a) aresubstantially identical and the shape of said first surface is definedby a function S(r, θ) determined by:${W_{1}\left( {r,\theta} \right)} \approx {\left( {n - 1} \right)\varphi\frac{\partial S}{\partial\theta}}$where “(r, θ)” are polar coordinates in a reference plane (X_(A)Y_(A))perpendicular to said optical axis (12), these coordinates beingcentered on the first point of intersection (A) of said axis of rotation(Z_(A)) and said reference plane, “W₁(r, θ)” is said wavefrontmodification expressed in the polar coordinates (r, θ), “n” is therefractive index of said first and second elements, “Φ” is said angle ofrotation, and “S(r, θ)” represents the shape of said first asphericsurface.
 4. The optical scanning device (1) as claimed in claim 2,wherein said functions S_(a)(r, θ) and S_(b)(r, θ) and/or S(r, θ)include(s): the term “(y+R)(x²+(y+R)²−yR)” for introducing saidwavefront modification W₁ in the form of coma along the X-axis, the term$``{{{- {R\left( {{2\left( {x^{2} + \left( {y + R} \right)^{2}} \right)} + R^{2}} \right)}}\mspace{14mu}{\arctan\left( \frac{y + R}{x} \right)}} - \left( {x^{2} + \left( {y + R} \right)^{2} + {3R^{2}}} \right) + {{Rx}\left( {y + R} \right)}}"$for introducing said wavefront modification W₁ in the form of coma alongthe Y-axis, the term “y+R” for introducing said wavefront modificationW₁ in the form of tilt along the X-axis, the term “x” for introducingsaid wavefront modification W₁ in the form of tilt along the Y-axis, theterm$``{{\left( {x^{2} + \left( {y + R} \right)^{2}} \right)\mspace{14mu}{\arctan\left( \frac{y + R}{x} \right)}} + {2{Rx}}}"$for introducing said wavefront modification W₁ in the form of defocus,the term$``{{\left( {x^{2} + \left( {y + R} \right)^{2}} \right)\mspace{20mu}\arctan\mspace{11mu}\left( \frac{y + R}{x} \right)} + {x\left( {y + R} \right)}}"$for introducing said wavefront modification W₁ in the form ofastigmatism along the X-axis, the term$``{{\left( {x^{2} + \left( {y + R} \right)^{2}} \right)\mspace{20mu}\arctan\mspace{11mu}\left( \frac{y + R}{x} \right)} - {x\left( {y + R} \right)}}"$for introducing said wavefront modification W₁ in the form ofastigmatism along the Y-axis, the term$``\left( {\left( {x^{2} + \left( {y + R} \right)^{2}} \right) + {4{R^{2}\left( {x^{2} + \left( {y + R} \right)^{2} + R^{4}} \right)}\mspace{14mu}{\arctan\left( \frac{y + R}{x} \right)}} + {4{R\left( {\left( {x^{2} + \left( {y + R} \right)^{2}} \right) + R^{2}} \right)}x} - {2R^{2}{x\left( {y + R} \right)}}}" \right.$for introducing said wavefront modification W₁ in the form of sphericalaberration, the term$``{{{- \frac{2}{3}}x^{3}} - {x\left( {y + R} \right)}^{2}}"$ forintroducing said wavefront modification W₁ in the form of line comaalong the Y-axis, or the term$``{{x^{2}\left( {y + R} \right)} + {\frac{2}{3}\left( {y + R} \right)^{3}}}"$for introducing said wavefront modification W₁ in the form of line comaalong the X-axis, where “(x, y)” are the Cartesian coordinates in thedirect orthogonal system X_(O)Y_(O) in said reference plane (X_(A)Y_(A))and having its origin on the second point of intersection (O) of saidoptical axis (12) and said reference plane, the Y_(O)-axis passingthrough said first point of intersection (A), and “R” is the distancebetween said first point of intersection (A) and said second point ofintersection (O).
 5. The optical scanning device (1) as claimed in claim2, wherein said functions S_(a)(r, θ) and S_(b)(r, θ) and/or S(r, θ)includes at least a step-function Q(r, θ) which equals: a nonzeroconstant parameter (q) for a portion of the corresponding asphericsurface, that parameter being substantially equal to mλ/(n−1) where “λ”is the wavelength of the radiation beam in the optical path of whichsaid wavefront modifier (30) is arranged, “m” is an integer value and“n” is the refractive index of the corresponding element, and zero forthe remaining part of that surface.
 6. The optical scanning device (1)as claimed in claim 1, wherein said axis of rotation (Z_(A)) iseccentric with respect to said optical axis (12).
 7. The opticalscanning device (1) as claimed in claim 6, wherein said axis of rotation(Z_(A)) is outside of the cross-section of the radiation beam incidenton said first and second elements (301, 302).
 8. The optical scanningdevice (1) as claimed in claim 1, wherein said detection system (10) isarranged for providing a focus error signal (S_(focus)) and/or aradial-tracking error signal (S_(radial)) and in that it furtherincludes a servo circuit (11) and an actuator (12, 13) responsive tosaid focus error signal and/or said radial-tracking error signal forcontrolling the positions of said scanning spot (17) with respect to theposition of said information layer (2) and/or of a track of saidinformation layer which is to be scanned.
 9. The optical scanning device(1) as claimed in claim 1, further including an information processingunit for error correction (14).
 10. The optical scanning device (1) asclaimed in claim 1, further including a wavefront compensator (19) whichincludes: an aberration detector (33) for providing a detection signal(35) representative of a wavefront distortion W₂ present in saidconverging radiation beam (16), and said wavefront modifier (30)arranged for, in response to said detection signal, introducing saidwavefront modification W₁ so that W₂+W₁=0.
 11. A wavefront modifier (30)for transforming a first radiation beam into a second radiation beam,the wavefront modifier having an optical axis (12) and including a firstelement (301) having a first aspheric surface (301 b) and a secondelement (302) having a second aspheric surface (302 a), said first andsecond elements being mutually movable in a plane perpendicular to saidoptical axis for introducing a wavefront modification (W₁) in saidsecond radiation beam, characterized in that said first and secondaspheric surfaces are shaped so that a mutual rotational displacement ofsaid first and second elements about an axis of rotation (Z_(A)) whichis parallel to said optical axis (12) generates said wavefrontmodification (W₁).